Optimal finger height for best finger control in dental work

Mansuang Wongsapai

Ph.D. Dissertation Protocol

Advisors

Ass.Prof. Dr. Siriwan Suebnukarn

Co-Advisors

Dr. Sunsanee Rajchagool

Faculty of Dentistry

ThammasatUniversity

June 2009

Program Authorized to Offer Degree: Oral Health Science

  1. INTRODUCTION

Background and significance

Human movement is a complex affair. Normal movements are often automatic in nature and only come under volitional control when circumstances change or as a consequence of new experiences. The unique functional ability of humans is possible through the quality of movement produced from normal coordinated patterns of movement. Such patterns are produced on a background of normal sensory information and feedback, normal tone, reciprocal innervation, normal balance and postural reactions (Edwards 1996).

The control of movement has traditionally been considered to be hierarchical with the highest, cortical level of control organising voluntary, skilled movement. However, in reality there is no separation between voluntary movements and the background of postural control that maintains the body in an upright position with the aid of automatic reflexes and responses. Therefore, parallel systems of control, with integration of all levels rather than just a serial hierarchy, may be a more appropriate description. All levels of control, from the spinal cord up to the cerebral cortex, are necessary and integrated to provide the base of axial stability for more normal distal mobility and skilled or refined coordinated limb movements (Kidd et a11992).

OVERVIEW OF MOVEMENT AND POSTURAL CONTROL

For any normal motor function to occur an input of sensory information is required. Sensory information is integrated at all levels of the nervous system and causes appropriate motor responses, i.e.:

• Spinal cord: simple reflexes (automatic, stereotyped reflex movement); the peripheral execution level of movement.

• Brain stem and basal ganglia: more complicated responses (postural and balance reactions), able to affect the spinal cord to produce or change automatic movement.

• Cerebrum: most complicated responses controlled (variable and adaptable skilled voluntary movement based on stored programmes of learned movements), The motor, sensory and associated areas begin the chain of commands for most movement, though this may be initiated or modified at lower levels.

• Cerebellum: planning, timing and predictive function to produce coordinated skilled and rapid movements.

Figure 1illustrates this movement control system with the interconnections between the main levels of the central nervous system suggesting a concurrent parallel organisation (Kidd et a11992).

Figure 1 : A schematic representation of the main control processes of voluntary movement.

ORGANISATION OF THE SPINAL CORD FOR MOVEMENT CONTROL

The grey matter of the spinal cord is the integrative area for the cord reflexes and other automatic motor functions. As the region for the peripheral execution of movements, it also contains the circuitry necessary for more sophisticated movements and postural adjustments. Sensory signals enter the cord through the sensory nerve roots and then travel to two separate destinations:

• Same or nearby segments of the cord where they terminate in the grey matter and elicit local segmental responses (excitatory; inhibitory; reflexes, etc.).

• Higher centres of the CNS, i.e. higher in the cord, and brain stem cortices where they provide conscious (and unconscious, i.e. cerebellum) sensory information and experiences.

Each segment of the cord has several million neurones in the grey matter that include sensory relay neurones, anterior motor neurones and interneurones. Interneurones are small and highly excitable with many interconnections, either with each other or with the anterior motor neurones. They have an integrative/processing function within the spinal cord as few incoming sensory signals to the spinal cord or signals from the brain, terminate directly on an anterior motor neurone. This is essential for the control of motor function. One specific type of interneurone is called the Renshaw cell, located in the anterior horn of the spinal cord. Collaterals from one motor neurone can pass to adjacent Renshaw cells which then transmit inhibitory signals to nearby motor neurones. So stimulation of one motor neurone can also inhibit the surrounding motor neurones. This is termed recurrent or lateral inhibition. This allows the motor system to focus or sharpen its signal by allowing good transmission of the primary signal and suppressing the tendency for the signal to spread to other neurones (Rothwell 1994).

In addition to interneurones there are also propriospinal fibres that run from one segment of the cord to another, so providing pathways for multisegmental reflexes, i.e. those reflexes that coordinate movement in different limbs simultaneously. Segmental circuits can activate networks of anterior horn cells and thus trigger the stimulation of specific muscle fibres. This can activate circuits (called central pattern generators) that control locomotion and possibly also a number of other repetitive motor activities (Marieb 1998).

BRAIN STEM AND BASAL GANGLIA LEVEL OF CONTROL OF POSTURE AND MOVEMENT

The principal role of the brain stem in control of motor function is to provide background contractions of the trunk, neck and proximal parts of limb musculature so providing support for the body against gravity. The relative degree of contraction of these individual anti-gravity muscles is determined by equilibrium mechanisms, with reactions being controlled by the vestibular apparatus, which is directly related to the brain stem region.

The brain stem connects the spinal cord to the cerebral cortex. It comprises the midbrain (mesencephalon), pons and medulla oblongata. The central core of this region is often referred to as the reticular formation. This region of the central nervous system comprises all the major pathways connecting the brain to the spinal cord in a very compact, restricted space. It is also the exit point of the cranial nerves from the central nervous system.

The central core of the brain stem region is the reticular formation. This is the collection of neurones (nerve fibres, etc. that form the ascending and descending pathways), the connections with the cerebellum and other parts of the brain, collateral fibres and the origin of some pathways. Therefore, the area functions as a relay station processing sensory information and organising motor output (Gordon 1990). The reticular formation influenced through the reticular activating system also controls alertness.

It is through the integration of the information reaching the reticular formation that axial postural control and gross movements are controlled. Input to the reticular formation is from many sources, including the spinoreticular pathways, collaterals from spinothalamic pathways, vestibular nuclei, cerebellum, basal ganglia, cerebral cortex and hypothalamus. The smaller neurones make multiple connections within the area whereas the larger neurones are passing through, being mainly motor in function.

The vestibular nuclei are very important for the functional control of eye movements, equilibrium, the support of the body against gravity andthe gross stereotyped movements of the body. The direct connections to the vestibular apparatus of the inner ear and cerebellum, as well as the cerebral cortex, enable the use of preprogrammed, background attitudinal reactions to maintain equilibrium and posture. Working with the pontine portion of the reticular formation, the vestibular nuclei are intrinsically excitable; however, this is held in check by inhibitory signals from the basal ganglia (Guyton 1991).

Overall, the motor related functions of the brain stem are to support the body against gravity, generate gross, stereotyped movements of the body and maintain equilibrium. Therefore, it is predominately concerned with the muscular control of the axial and proximal limbs (trunk and girdle movements). This is not in isolation but assisted by the integration of information from the cerebellum, basal ganglia and cortical regions.

The brain stem influences motor control directly through the descending pathways of the spinal cord and indirectly through ascending pathways to higher centres where their role is controlling overall activity of the brain and so of alertness (Gordon 1990). The descending pathways, which help control axial and girdle movements, can be divided into medial and lateral motor systems.

The medial system descends in the anteromedial columns of the spinal cord and projects directly (through interneurones) to medial motor neurones of the anterior horn, influencing groups of proximal limb muscles and axial body regions, often bilaterally. It is important for organising and controlling whole body movements that require groups of muscles working together. The vestibulospinal pathways are specifically related to the position and movement of the head, making them important in the organisation of postural movements in balance control. The reticulospinal pathways influence postural movements and locomotion through pontine portions that are inhibitory (Guyton 1991), so facilitating the support of the body against gravity by exciting the anti-gravity muscles. The tectospinal pathways link the midbrain with the cervical and upper thoracic spinal cord and are important for Organising and orientating movements of the head and neck.

The lateral system descends in the dorsolateral columns of the spinal cord and directly or indirectly innervates the motor neurones to the distal muscles of the limb, i.e. it is for more discrete muscle actions that are concerned with the individual, agile and skilled movements of the extremities (Kidd et al 1992). The brain stem is probably involved in the circuits that produce the central pattern generators that coordinate locomotion. The impulse for walking may come from higher cortical centres but these central pattern generators are able to provide the motor pattern for walking (Grillner et al 1955)

The basal ganglia region forms part of the internal structure of the cerebral hemispheres. It comprises five subcortical nuclei (the putamen; caudate nucleus; globus pallidus; subthalamic nucleus and substantia nigra) which seem to serve as side loops to the cerebral cortex as they receive their input from the cerebral cortex and project almost exclusively back to the cerebral cortex. The basal ganglia are involved in all types of movements but have a predominant role in the provision for internal cues for the smooth running of learned movements and in the maintenance of the preparedness for movement (Morris and Iansek 1996). It is believed that the basal ganglia play an essential role in the selective initiation of most activities of the body or selective suppression of unwanted movements. A number of distinct feedback loops have been described including the putamen circuit (the direct pathway), which is responsible for the facilitation of movement, and the caudate circuit (the indirect pathway), which is more involved in the inhibition of unwanted movements. Thus the interplay of inhibitory or excitatory neurotransmitters within this region explains the clinical features that emerge in disorders of the basal ganglia (Rothwell 1994).Figure 2 shows a hypothetical model of the direct pathway. Excitation of the striatum from areas of the cerebral cortex inhibits the globus pallidus (internal part) and thus reduces the inhibitory influences of this area upon the thalamus. This in turn disinhibits the thalamus to excite areas of the cerebral cortex for the initiation of movement. The normal functioning of this loop is reliant upon excitatory neurotransmitter impulses from the substantia nigra.

Figure 2 : Hypothetical model of the direct pathway in the basal ganglia

CEREBRAL CORTEX (CORTICAL) CONTROL OF MOVEMENT

Posture and equilibrium are controlled subconsciously by the brain stem and spinal cord; however, the cerebral cortex is the main centre for the control of voluntary movement. It works with the information it receives from the cerebellum, basal ganglia and other centres in the CNS to bring movements under voluntary control.

The cerebral cortex provides the advanced intellectual functions of humans, having a memory store and recall abilities along with ,other higher cognitive functions. The cerebral cortex is, therefore, able to perceive, understand and integrate all the various sensations. However, its primary movement function is in the planning and execution of many complex motor activities, especially the highly skilled manipulative movements of the hand (Gilman & Newman 1987).

The motor cortex occupies the posterior half of the frontal lobes. It is a broad area of the cerebral cortex concerned with integrating the sensations from the association areas with the control of movements and posture. It is closely related to other motor areas including the primary motor area and the premotor or motor association area.The primary motor area contains very large pyramidal cells that send fibres directly to the spinal cord and anterior horn cells via the corticospinal pathways. In contrast, the premotor area has only a few fibres connecting directly with the spinal cord. It mainly sends signals into the primary motor cortex to elicit multiple groups of muscles; i.e. signals generated here cause more complex muscle actions usually involving groups of muscles that perform specific tasks, rather than individual muscles. This area connects to the cerebellum and basal ganglia which both transmit signals back, via the thalamus, to the motor cortex. Projection fibres from the visual and auditory areas of the brain allow visual and auditory information to be integrated at cortical level to influence the activity of the primary motor area. The premotor area is activated when a new motor programme is established or when the motor programme is changed on the basis of sensory information received, i.e. exploring a new environment. The supplementary motor area is thought to be the site where external inputs and commands are matched with internal needs and drives to facilitate formulation (programming) of a strategy of voluntary movement. So altogether the motor cortex and related areas of basal ganglia, thalamus and cerebellum constitute a complex overall system for voluntary control of muscle activity (Afifi Bergman 1986).

The primary motor area has a topographical representation of the body on the surface of the cerebral cortex. This demonstrates the connections of the cerebral cortex to the different areas of the body with the size of the representation being proportional to the degree of innervation in that area. The hands are exaggerated in form as the refined skilled movements produced by the hand require more innervation to achieve that level of control (Palastanga et al 1994).

For voluntary movement to occur, information has to reach the motor cortex, mainly from the somatic sensory systems plus auditory and visual pathways. The sensory information is processed with information from the basal ganglia and cerebellum to determine an appropriate course of action. As can be seen there are many two-way pathways between the various centres of the CNS.

The organisation of the motor cortex is in vertical columns which act as functional units. Information enters a column from many sources and is amplified as necessary to produceappropriate muscle contraction. Each functional unit is responsible for directing a group of muscles acting on a single joint. So movements, not individual muscles, are represented in the motor cortex. Individual muscles are represented repeatedly, in different combinations, amongst the columns. Neurones of the motor cortex, having axons in the corticospinal pathways, function chiefly in the control of the distal muscles of the limb. They function to:

  • change their firing rate in advance of limb movements
  • fire at a frequency that is proportional to the force to be exerted in a movement and not in relation to the direction of the movement (Gilman 1992).

Each time the corticospinal pathway transmits information to the spinal cord the same information is received by the basal ganglia, brain stem and cerebellum. Nerve signals from the motor cortex cause a muscle group to contract. The signal then returns form the activated region of the body to the same neurones that caused the contraction, providing a general positive feedback enhancement if the movement was successful and recording it for future use.

At any segment of the spinal cord multiple motor pathways enter/terminate in the cord from the brain stem or higher centres. Generally, the corticospinal and rubrospinal pathways lie in the dorsal portions of the lateral columns of the spinal cord and terminate on the interneurones when concerned with the trunk, leg and arm areas of the cord. However, at the cervical enlargement where the hands and fingers are represented, the motor neurone supplying the hands and fingers lies almost entirely in the lateral portions of the anterior horns. In this region a large number of corticospinal and rubrospinal fibres terminate directly onto the anterior horn cells, i.e. a direct route from the brain in keeping with a high representation for fine control of the hand. fingers and thumb in the primary motor cortex (Ganong 1995).